Being one of the deadliest contagious diseases worldwide, tuberculosis (TB) is accounted as one of the threatening health concerns. Although tuberculosis is an ancient disease, money and efforts are still invested to make TB a history [1]. Based on the 2022 WHO global report of tuberculosis, it is expected that TB will replace COVID-19 as the leading cause of death from an infectious agent on a global level [2]. Yet, first- and second-line agents in addition to few newly approved agents are indicated to treat TB through prolonged regimens. The complicated and overlong treatment regimens are considered a true challenge to eradicate tubercular disease. Moreover, the pervasiveness of multidrug-resistant tuberculosis and extensively drug-resistant tuberculosis (MDR-TB and XDR-TB, respectively) turned these treatment options to be cumbersome [3]. According to the WHO, the incidence of TB is expected to be increasing with an estimate of 500 000 new cases of MDR-TB to emerge each year [4]. Determined efforts are being exerted to attain an effective anti-tubercular drug against sensitive and resistant TB strains. The anticipated drug is requested to shorten the treatment duration and to improve patient compliance as well. The aforementioned facts emphasize the necessity to contrive novel agents outpacing current ones through improving efficiency and encountering the resistance developed.

Benzo[b]thiophene is a drug-like fragment with an immense pharmacological potential [5], [6], [7]. It is considered one of the crucial heterocycles with diverse biological activities as anti-cancer [8], [9], [10], anti-microbial [11], [12], [13], [14], [15], [16] and antimalarial [17], [18], [19]. Benzo[b]thiophene was investigated for its anti-mycobacterial activity and the potency of the developed agents attracted attention towards the possibility of having novel anti-tubercular agents that incorporate benzo[b]thiophene nucleus [5], [20]. Compound I, is a benzo[b]thiophene-based compound that was synthesized and evaluated for its anti-mycobacterial activity and it showed up an eye-catching activity against active and dormant MDR-TB with MICs 5.3 and 2.73 µg/mL, respectively [21] (Fig. 1). Moreover, a benzo[b]thiophene-quinazolinone hybrid (compound II) revealed a remarkable anti-tubercular activity with MIC of 6.25 µg/mL [22].

On another level, isatin is a privileged building block with versatile in vitro and in vivo activities targeting many diseases including cancer, viral and microbial infections [23], [24], [25], [26], [27], [28], [29], [30], [31]. Isatin hybrids based on tethering with other moieties were also reported for their potent anti-tubercular activities [32], [33], [34], [35], [36], [37], [38], [39]. N-acylhydrazone is one of the abundant linkers in medicinal compounds including anti-tubercular agents and it is noteworthy that N-acylhydrazone was seen as a bioactive linker in many of these anti-tubercular hybrids. This is exemplified in Fig. 1 where compounds III and IV show encouraging anti-tubercular activity based on the isatin-N-acylhydrazone scaffold (MICs = 25 and 0.24 µg/mL, respectively) [40], [41].

In view of the aforementioned findings and in continuing our efforts to develop novel agents with potent anti-tubercular activity, we report in this study the synthesis and biological evaluation of novel benzo[b]thiophene-isatin hybrids that are connected through N-acylhydarzone linker. Since lipophilicity is of great impact in the development of effective anti-tubercular agents, two series of 3-un/methylsubstituted benzo[b]thiophene derivatives were synthesized releasing compounds 6a-i and 7a-i, respectively. Being a pillar in anti-TB drug discovery researches and based on the potency shown by compound IV, isatin was hybridized with 3-un/methylsubstituted benzo[b]thiophene and the molecules were assembled through the eminent N-acylhydarzone linker (Fig. 1). To further widen the scope of our design, isatin ring was adapted for different substituents. Besides, the effect of changing the position of theses substituents around C-5, C-6 and C-7 was investigated. The synthesized compounds were screened versus the Mycobacterium tuberculosis drug-sensitive strain (ATCC 25177). Eventually, compounds with considerable activities were distinguished and promoted for testing their anti-mycobacterial potency against resistant strains MDR-TB (ATCC 35822) and XDR-TB (RCMB 2674).